The present invention relates to contact films, or more particularly to contact films which do not contain silver halide compounds as part of their light sensitive coating.
Conventionally, in the preparation of light sensitive lithographic printing plates, an image is imparted to such plate via contact exposure or projection through a photographic mask. Such a mask typically comprises a transparent support and a photosensitive composition on the transparent support. This photosensitive composition is typically exposed to an original image sought to be reproduced, and developed. The vast majority of such photomasks employ silver halide gelatin compositions on the support as the imaging agent. With the recent dramatic increases in the cost of silver and its reclamation, users of such films have desired a film having the benefits of silver halide films yet which do not contain silver. Although this has been a goal of the art and many substitutes have been proposed, these have not been satisfactory to those skilled in the art of printing plate manufacture and use. These proposals are based upon diazo-coupler or photopolymer systems as the means of creating a UV activated, developer differentiated imaging method. These systems have severe shortcomings in the areas of proper light transmission density, exposure speed, speed of development, developer properties and opacity to UV radiation after exposure and development. Such films therefore find their usefulness primarily as proofing or duplicating films and not as high density films which may be used to expose printing plates as they are aesthetically unsatisfactory due to their color and high transmission of visible light.
The present invention overcomes these problems to provide a photographic element which has sufficient light sensitivity to be exposable within a commercially feasible time; is developable with a fluid developer so as to quickly and completely remove the non-image areas; and possesses sufficient opacity in the non-removed areas so it will be useful to block sufficient ultraviolet radiation when used as a mask to further expose lithographic printing plates or other photographic elements.
Exposure masks, or contact flats as they are more commercially called, are either negative or positive working. A negative flat is characterized by having what corresponds to the background on the printed copy being covered witb the opaquing emulsion while what will be the image is free of emulsion and therefore clear. A positive flat is the reverse. The image area is opaqued whereas the background is clear.
Independent of the means by which the image and non-image are differentiated, the opaquing emulsion must have sufficient density so as to prevent the transmission of light and therefore the unwanted exposure of the light sensitive coating being opaqued. Depending on the sensitivity or light speed of the coating, a greater or lesser density may be used to effectively prevent unwanted exposure. Density is measured as a logarithmic function. For example, a density of 1.0 means that 90.0% of the incident light is opaqued, 10.0% is transmitted. A density of 2.0 means that 99.0% of the incident light is opaqued and 1.0% is transmitted. Generally, a density of 3.0% is accepted as a minimum, wherein 0.1% of the incident light is transmitted.
To expose a plate or any recording medium, the exposure flat must first be properly imaged. This is normally done by placing the film which is to be the exposure flat in a camera and taking a picture of the image to be replicated, often called a paste-up.
The exposed film is then properly developed. This produces a negative flat unless a reversed image was originally used to make the camera image on the film. When a positive flat is required, the negative flat is used as the dummy sheet to make another exposure to produce the positive image. This film is likewise developed.
The developed and dried film is placed on the coated surface of the plate or recording medium in a fashion such that the emulsion side of the flat is in contact with the coating. A vacuum is drawn to remove all air between tbe coating and the emulsion of the exposure flat. A good contact ensures a one-to-one image ratio, minimal halation, and no "hot-spots."